Option to not display obsolete gems

[Lucivaldo Castro]

We could add an attribute in gemspec, which indicates whether the gem is obsolete or not. This information should be updated when the developer leaves the project and decides not to update the code any more. In this way, the command gem list <gem name> --remote would have one more option to not display the gems that are obsolete, that it does not make sense to display them in many situations, improving the developer's search for gems that are compatible with the current codes.